Balmer line emission from the accretion disks surrounding the supermassive black holes in active galactic nuclei

The hydrogen Balmer emission lines are the most prominent feature in the optical spectra of low redshift Active Galactic Nuclei (AGN). Their line widths or the Balmer-to-metal line flux-ratios are used to distinguish active from star-forming galaxies. Yet the geometry and dynamics of the gas responsible for these lines are still poorly constrained. Using a large new sample of a rare class of broad-line AGN with double-peaked Balmer-line profiles indicative of an accretion disk origin, I consider the contribution of disk emission to Hα, the strongest of the observed Balmer lines. I focus on finding an efficient method for the selection of double-peaked AGN, which constitute about 3% of all AGN with z < 0.33 discovered with the Sloan Digital Sky Survey. I proceed to define the characteristic properties which distinguish the double-peaked AGN from the general population, searching for the underlying reasons for their strong disk-emission contribution. Using both statistical comparison of line shapes and direct disk-model fitting methods, I study the sizes, inclinations, and surface emissivity profiles of accretion disks in the light of their Hα line emission. I find that 60%–80% (depending on the strength of the asymmetry) of all observed line-shapes originate in non-axisymmetric disks, and that single-epoch observations of the Hα line profiles are not sufficient to constrain the surface emissivity patterns of the accretion disks. With the help of theoretically motivated surface emissivity laws, it is possible to infer the distribution of inclinations and accretion disk sizes. I find evidence for the existence of obscuring material coplanar with the disks at larger distances from the central black holes, supporting unification theories for broad and narrow line AGN. The sizes of Hα emitting rings implied by the observations suggest that they are likely limited by the onset of self-gravity, a few thousand gravitational radii from the center. The most probable causes for the small accretion disk emission contribution to the Balmer line flux in the general AGN population are the onset of self gravity, which fragments and destroys the outer parts of the disk, and/or the lack of external illumination necessary to produce detectable accretion disk contribution.